1. Field of the Invention
The present invention relates to an image development device for use with an electrostatic latent image carrier, and more particularly to a magnetic brush image development device for use in an electrophotographic copying machine, an electrostatic printing machine, or other recording apparatus.
2. Description of the Prior Art
Conventional magnetic brush development devices generally have a plurality of permanent magnets fixedly disposed in a nonmagnetic sleeve, with adjacent permanent magnets having opposite polarities. When the sleeve rotates, a two-component developer composed of toner and a magnetic carrier is delivered as a magnetic fiber brush on the sleeve toward an electrostatic latent image carrier. Since the developer attached to the sleeve is too much, its amount is limited by a limiting plate called a doctor blade.
One such conventional magnetic brush development device is hereinafter explained with reference to FIG. 21 of the accompanying drawings.
As shown in FIG. 21, a photosensitive drum 1 serving as an electrostatic latent image carrier is driven to rotate about its own axis in the direction of the arrow. A magnetic brush development device 2 disposed in confronting relation to the photosensitive drum 1 comprises a development sleeve 3, a development casing 4, a doctor blade 5, magnets 6, a toner fiber limiting plate 7, an inlet seal member 8, and other members.
The development sleeve 3 comprises a nonmagnetic cylindrical body having its peripheral surface close to the photosensitive drum 1 and being rotatable so that its portion facing the photosensitive drum 1 moves in the same direction (indicated by the arrow) as the direction in which the confronting portion of the photosensitive drum 1 moves. The development sleeve 3 houses therein a group of magnets such as the magnets 6, a development main pole 6a, delivery magnets 6b, 6c, etc, which are fixed in position.
The doctor blade 5 is positioned closely to the peripheral surface of the development sleeve 3. The doctor blade 5 serves to limit the amount of a developer moving with the development sleeve 3--i.e., the amount of the developer to be supplied to the photosensitive drum 1. The toner fiber limiting plate 7 is located downstream of the doctor blade 5 in the direction of travel of the developer. The toner fiber limiting plate 7 covers as a roof-like member a portion of the peripheral surface of the development sleeve 3 for holding down toner fibers of the developer on the peripheral surface of the development sleeve 3 thereby to preventing the developer from being scattered. The doctor blade 5 and the toner fiber limiting plate 7 are integrally molded as a single-piece doctor member 9. The inlet seal member 8 is positioned above the doctor member 9 and has a distal end extending toward the photosensitive drum 1. A brush 8a of bristles attached to the distal end of the inlet seal member 8 in any known manner is held against a portion of the peripheral surface of the photosensitive drum 1 for preventing the toner from being scattered out of the magnetic brush development device 2. A scoop-up roller 10 for supplying the developer is disposed in confronting relation to the development sleeve 3. The scoop-up roller 10 has a number of radial blades for retaining the developer therebetween and supplying the developer to the development sleeve 3 upon rotation in the direction of the arrow.
The development sleeve 3 and the scoop-up roller 10 are rotatably housed in the development casing 4, which has an opening facing the photosensitive drum 1. The development casing 4 also has an opening defined in an upper righthand portion thereof and coupled to a toner supply tank 11. The toner supply tank 11 supplies toner T to the scoop-up roller 10 in response to rotation of a supply roller 12 in the toner supply tank 11. The toner T can be supplied to the toner supply tank 11 from a toner hopper 13. The toner T within the toner supply tank 11 is agitated by an agitator 14 at all times.
The toner T supplied by the scoop-up roller 10 to the development sleeve 3 in the development casing 4 is mixed with magnetic particles known as a carrier, and then forms a magnetic brush under magnetically attractive forces from the delivery magnets 6b, 6c. The magnetic brush is moved toward the photosensitive drum 1 as the development sleeve 3 is rotated.
A first space A is surrounded by the doctor blade 5, the toner fiber limiting plate 7, and a portion of the peripheral surface of the development sleeve 3. A second space B is surrounded by the inlet seal member 8, the toner fiber limiting plate 7, a portion of the peripheral surface of the development sleeve 3, and a portion of the peripheral surface of the photosensitive drum 1. Air flows between the first and second spaces A, B as follows: When the development sleeve 3 rotates in the direction of the arrow, the developer that has been held down by the toner fiber limiting plate 7 serves as a pump for delivering air from the first space A into the second space B. As air is thus delivered, the air pressure in the second space B is increased, and higher-pressure air in the second space B tends to flow through a portion of the inlet seal member 8 with a less sealing capability out of the development casing 4.
The above problem is solved by a magnetic brush development device disclosed in Japanese Laid-Open Patent Publication No. 59-101676. The disclosed development device has a pump for drawing air out of a space corresponding to the second space B, thereby preventing an air-pressure buildup in that space. However, the proposed arrangement is disadvantageous in that it not only requires a pump but also needs a relatively large area to install the pump therein.
In the unitized development device as described above, an air flow (hereinafter referred to as a "wake") is produced around the scoop-up roller 10 and the development sleeve 3 as they are rotated. The wake increases the air pressure within the development casing 4, and it causes some of the toner to be scattered out of the development unit on an air stream flowing from the development casing 4.
The toner is mainly apt to flow out of the development unit through a gap between the development sleeve 4 and the development casing 3. In general, no magnetic brush is normally formed at each end of the development sleeve 3. Therefore, the gap between the development casing 4 and each end of the development sleeve 3 is larger, as no magnetic brush toner fiber is present at each end of the development sleeve 3. As a consequence, toner particles tend to be forced through the opposite ends of the development sleeve 3 and scattered out of the development unit.
When the developer is scattered out of the development unit, it floats in the recording apparatus and contaminates various other components therein. If an image transfer charger is contaminated by such floating developer particles, a defective image may be produced.
To prevent the developer from being scattered out through the gaps at the opposite ends of the development sleeve 3, many development device designs have seal members attached to the development casing 4 in covering relation to the opposite ends of the development sleeve 3. However, floating toner particles are liable to be deposited on the seal members, and such a toner deposit is caused to drop by an air flow produced by the rotation of the photosensitive drum 1, thus scattering toner particles around. It would be difficult to form a suitable gap between the outer end of the magnetic brush on the development sleeve 3 and the inner edges of the seal members. Thus, the magnetic brush is displaced by the seal members to cause the toner to be scattered out and drop onto the image transfer charger, for example, disposed below the development device, resulting in a leakage or other failure. Alternatively, the outer end of the magnetic brush and the seal members may be spaced too apart from each other, failing to prevent the toner from scattering out.
One solution to the above problem has been to define a pressure relief hole in the development casing 4 and provide a filter in the pressure relief hole for preventing the developer from flowing out therethrough. The conventional filter structure is however disadvantageous in that it tends to be clogged soon, quickly reducing the pressure relieving capability thereof.